1. Field of the Invention
The present invention relates generally to a three-dimensional (3D) shape measurement apparatus and method using the stereo moiré technique, and, more particularly, to a 3D shape measurement apparatus and method using the stereo moiré technique, in which a stereo vision technique using two cameras is combined with existing Phase Measuring Profilometry (PMP) technique and a novel stereo phase analysis method is applied, thereby solving the 2π ambiguity problem of typical moiré technique.
2. Description of the Related Art
With regard to 3D shape measurement technology, optical trigonometry using a laser structured light, a measurement method using a stereo vision technique, and a measurement method using the moiré principle have been developed and used. Among these methods, a 3D shape measurement method using the moiré principle has been widely used, and, more particularly, PMP has been more popularly used. Since these measurement methods are based on optical methods, they can be used for fast and accurately acquiring 3D shapes of object.
However, the 3D shape measurement method using typical moiré technique has inherent limitation: 2π ambiguity that makes impossible to find a correct object's shape measurement. If object has the height over than 2π, which is determined by the period of a periodic pattern projected by pattern projector, PMP technique cannot measure correct object shape. Accordingly, many methods for solving this 2π ambiguity problem have been conducted such as unwrapping method and phase shifting method. However, these methods based on typical moiré technique still have the problem of causing inaccurate results, because it is difficult to determine whether a measurement result value is related to 2π ambiguity or is related to objects' shape to be measured, having a depth equal to or greater than 2π, in the case where two objects to be measured are spaced apart from each other by a distance equal to or greater than 2π. That is, since typical moiré technique has the 2π ambiguity problem, it is possible to acquire relative 3D shape information of object, but it is impossible to acquire accurate 3D information. Accordingly, in the past, there were attempts to overcome the 2π ambiguity problem.
Of these attempts, Korean Unexamined Patent Publication No. 2005-0031328 disclosed “3D Inspection Method and Apparatus using Stereo Vision and Moiré.” This disclosed technology is configured such that, when an optical projection unit projects a uniform pattern a plurality of times and an object to be measured is phase-shifted using an actuator, the object to be measured is measured using two cameras at every time point, and the 3D shape information of the object to be measured is finally measured by comparing a plurality of pieces of 3D shape information, measured as described above, with each other. However, this technology could not completely overcome the 2π ambiguity problem either. That is, 2π ambiguity occurs at locations corresponding to multiples of right and left phase information. Furthermore, this technology could not overcome the problem of finding corresponding points.
Furthermore, research that attempted to measure a 3D shape using a pattern projector and two cameras was conducted (refer to “An active trinocular vision system of sensing indoor navigation environment for mobile robots,” authored by M. Y. Kim and H. S. Cho, and published in Sensors & Actuators: A. Physical, Vol. 125, No. 2, pp. 192-209, in 2006). This technology is configured such that a virtual pattern image is created with the pattern projector regarded as one virtual camera, and a 3D coordinate system is acquired using this image and the two remaining images captured from two cameras. In more detail, the 3D coordinate system is acquired through line matching between two captured images. However, this technology has problems in that the 2π ambiguity problem cannot be completely removed because it uses line matching. In addition, a long operating time is required for line blobbing.